1. Field of the Invention
This invention relates to a laser drive method and a laser drive system for controlling a light output of a laser diode pumped solid state laser, and more particularly to such a method and a system for restoring the light output of the laser diode pumped solid state laser upon receipt of a predetermined command. This invention further relates to a radiation image read-out apparatus provided with a stimulating light source driven by the laser drive system.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a laser diode pumped solid state laser in which a solid state laser crystal doped with a rare-earth element such as neodymium is pumped by a light emitted from a laser diode. In such a solid state laser, it has been widely put into practice to dispose a nonlinear optical crystal in the resonator thereof in order to convert a solid laser beam (a laser beam emitted from the solid state laser crystal) to a second harmonic or a sum frequency, thereby obtaining a laser beam of a shorter wavelength. For example a solid state laser in which a second harmonic is taken out is generally referred to as "a SHG laser".
In the solid state lasers including those in which the wavelength of the laser beam emitted from the solid state laser crystal is converted, the temperatures of the laser diode, the solid state laser crystal and the resonator are generally controlled to predetermined temperatures in order to suppress fluctuation in the light output and the oscillation wavelength of the laser diode and further to keep a predetermined phase matching state at the nonlinear optical crystal in the case of the lasers where the wavelength conversion is effected. Such temperature control is generally effected by placing those elements on cooling surfaces of electronic cooling elements (Peltier elements), detecting the temperatures of the laser diode and the inside of the resonator and feedback-controlling the electronic cooling elements on the basis of the detected temperatures so that the temperatures of the cooling elements are kept at target temperatures. In order to keep constant the light output of the solid state laser, there generally carried out an automatic power control in addition to said temperature control.
Further since the solid state lasers are suitable for obtaining a high light output, they have been increasingly used as high power lasers. For example, they are used as a stimulating light source in a radiation image read-out apparatus disclosed, for instance, in U.S. Pat. Nos. 4,584,482 and 5,530,261, due to their high output powers. The radiation image read-out apparatus comprises a stimulating light projecting means which projects stimulating light onto a stimulable phosphor sheet on which a radiation image has been stored and a read-out means which detects light emitted from the stimulable phosphor sheet upon exposure to the stimulating light in proportion to the radiation energy stored thereon and reads out the radiation image stored on the stimulable phosphor sheet.
There is a phenomenon that the light output power of the solid state laser gradually reduces with time though the automatic power control is carried out. The reason of this phenomenon has not been found.
When the solid state laser is used as the stimulating light source in said radiation image read-out apparatus, reduction in the light output power of the stimulating light source directly leads to deterioration in the image quality and accordingly the phenomenon governs the performance of the apparatus. Therefore, not only search for the reason of the phenomenon has been made but also how to deal with the phenomenon has been searched.